According to a known water treatment system for denitrifying nitrogen components in the water which are an important cause of water eutrophication, a part of the treated water passed through an aerobic treatment vessel is returned to an anaerobic vessel on the upstream so that the nitrogen components in the water which have been nitrified in the aerobic treatment vessel with the aid of nitrifying microbes are gasified in the anaerobic vessel on the upstream with the aid of heterotrophic denitrification microbes.
However, according to such a conventional water treatment system, an aeration process of a long time duration in the order of 10 hours is necessary for the nitrification of ammonia nitrogen in the aerobic treatment vessel on the downstream end. Also, a resident time of four to five hours is necessary for the denitrification to be completed in the anaerobic treatment vessel on the upstream end. As such, the system tends to be undesirable large in size, and requires a large floor area. Furthermore, when the water to be treated lacks organic carbonaceous components which are necessary for the denitrification process, it is necessary to add carbonaceous materials such as methanol to the water. For re-circulating or recycling the water which has been treated in the aerobic treating vessel back to the anaerobic treatment vessel, facilities such as piping arrangements and pumps are required, and it tends to undesirably complicate the system.